We have continued our combined biophysical and biochemical research on the molecular mechanisms of membrane fusion during exocytosis, the critical step in insulin, neurotransmitter, and immunoglobulin secretion, and in fertilization. Cortical vesicles (CV), the docked, release-ready, secretory organelles of sea urchin are isolated from concomitant processes such as vesicle mobilization from reserve pools and vesicle recycling, have all the factors essential to tight membrane-membrane contact, retain their capacity for Ca2+-triggered fusion in vitro, and are available in quantities sufficient for biochemical detection of membrane components. Our previously described model for multiple fusion complexes was used to analyze homotypic CV fusion, revealing that CV-CV fusion captures all the essential features of exocytosis, including the same number of operationally defined 'Fusion Complexes' per vesicle, and a similar Ca2+ sensitivity. CV-CV fusion is therefore a reduced model system for Ca2+-triggered exocytosis. Analysis of results with Sr2+ and Ba2+ also indicate that these divalent cations trigger fusion via the same common pathway as does Ca2+. Although SNARE complex disruption is Ca2+-triggered in this native membrane, disruption occurs at higher [Ca2+]free than is Ca2+-triggered fusion initiated. Furthermore, the Ca2+ sensitivity of the complex was unchanged in fusion-incompetent CV. Sr2+ and Ba2+ substituted for Ca2+ in triggering fusion but had no significant effect on complex disruption. Thus, contrary to current hypotheses in the literature, neither the formation, presence nor disruption of the SNARE complex is essential to fusion. We have also demonstrated this result for the fusion of endogenously docked CV to the plasma membrane. Thus, using both an experimental and a modeling approach, we find that the SNARE complex is not essential to the final steps of exocytosis. By exposing CV to Ca2+ while bringing them into contact (rather after the establishment of contact), results were obtained suggesting that inter-membrane SNARE complexes may enhance the Ca2+ sensitivity of the fusion reaction. Other membrane components or other SNARE protein functions, which must be essential for Ca2+-triggered membrane fusion, will be investigated next.